Fumed silica is generally produced by the vapor phase hydrolysis of chlorosilanes, such as silicon tetrachloride, in a hydrogen/oxygen flame. The overall reaction is: EQU SiCl.sub.4 +2H.sub.2 +0.sub.2 .fwdarw.SiO.sub.2 +4HCl
In this process, submicron sized molten spheres of silica are formed. These particles collide and fuse to form three dimensional branched, chain-like aggregates, of approximately 0.1 to 0.5 micron in length. Cooling takes place very quickly, limiting the particle growth and ensuring that the fumed silica is amorphous. These aggregates in turn become mechanically entangled to form agglomerates ranging in size from 0.5 to 44 micron (325 US mesh) in size. Generally, fumed silicas have B.E.T. (Brunauer Emmet Teller determined) surface areas ranging from about 50 to about 400 square meters per gram (sq. m/g) and bulk densities ranging from about 10 to about 50 g/l. Fumed silicas generally have very high purity, with total impurities, in many cases below 100 ppm.
Fumed silica produced by pyrogenic processes is characterized by having a high air content and a correspondingly low bulk density. The fumed silica is also hydrophilic, absorbing moisture from the atmosphere and thereby changing the flow and handling characteristics of the silica. Automated processes which utilize hydrophilic silica, require a uniformly moisturized fumed silica with predictable flow characteristics in order to diminish product variation and diminish maintenance and equipment costs. In certain applications such as in the production of 1 part RTV silicone rubber, water serves as catalyst in the curing stage. Non-Uniform distribution of water results in product variations. Uniform moisturization is also important to powder handling because it suppresses static charge buildup. Variation in moisture uniformity result in flow irregularities and contamination due to incomplete evacuation in equipment lines and reaction containers.
Fumed silica when exposed to high moisture environments and ambient temperatures (23.degree. C.) shows a maximum of about 2% to 8% physical adsorption of water. Adsorption levels however, may rise further depending on the temperature, humidity and duration of storage. By combining water with fumed silica in high energy mixing, it has been shown that particulate silica has the potential for adsorbing about 80-90 weight percent water. See U.S. Pat. No. 4,008,170, Brunner German Pat. No. 1,467,023, and Schutte U.S. Pat. No. 3,393,155. The moisturized silica, commonly termed "dry water" has a variety of applications including a cooling source to minimize the risk of explosion in rocket fuels or in the extreme temperatures of outerspace where temperatures of -196.degree. C. do not affect the flowability of the moisturized silica.
Fumed silicas can be treated with chemical agents to produce hydrophobic fumed silicas. Hydrophobic fumed silicas have utility in many applications. For example, hydrophobic fumed silicas may be used for reinforcement and rheology control fillers in RTV-2 (2 component, room temperature, vulcanized) type silicone compounds. Hydrophobic fumed silicas may also be used to provide corrosion resistance and hydrophobicity in coatings. Further, hydrophobic fumed silica acts as a free flow agent for powdered materials to prevent caking. For example, hydrophobic fumed silicas may be utilized in fire extinguishing powders, powdered polymers, pigments, toner, herbicides and insecticides. Still further, hydrophobic fumed silicas may be utilized to provide theology control and water resistance in polyester resins, in insulation coatings for moisture sensitive applications, for anti-settling purposes in aerosol paints, and for reinforcement and water repellency in dental compounds.
Hexamethyldisilazane (HMDS) may be used as a chemical treating agent to produce HMDS treated fumed silicas. A disadvantage of using HMDS is that HMDS produces ammonia as a by-product. The ammonia by-product is normally absorbed on the surface of the treated fumed silica. If such a product is put into part A of a RTV-2 system, the absorbed ammonia will cause Part A to have a high initial viscosity which increases during storage. Ammonia has been removed from treated silica by various methods, including heating.
Fumed silicas have also been treated with commonly known chemical agents such as dimethyldichlorosilane to produce a hydrophobic silica, and then combined with water, a wetting agent, and high shear mixing to produce an aqueous hydrophobic dispersion of silica. See U.S. Pat. No. 4,274,883. The references also discloses that a homogeneous dispersion of hydrophobic fumed silica was achieved without a wetting agent through prolonged mixing. The bulk density according to DIN 53194 was about 50 g/l.
The process for producing dry silica for use in a matting material, such as that used in polyurethane coatings having aerogel like structure including bulk densities of about 20 to 50 g/l and high macroporosity (DBP numbers up to 3.4 ml/g) is also known. See U.S. Pat. No. 4,150,101 where a pyrogenically derived fumed silica is combined with about 5 to 20% water and alternatively a pH adjustor followed by drying. The water is uniformly distributed in the silica by spraying the water into the stirred silica or by spraying the water into a fluidized flowing mass of silica by for example, a gravity pipe (fall tube). Steaming the water laden silica for a short period to enhance distribution of water is also disclosed. Applicant has determined however, that the application of steam to hydrophilic fumed silica produces a product in which the distribution of water is non-uniform.
Mixing carbon black with hydrophobic treated fumed silica has been used to improve the surface cure of vulcanizable compositions and reduce the "rub off" or removal of black from the surface material. See U.S. Pat. No. 4,221,693. Carbon black and the hydrophobic filler were mechanically agitated to obtain a homogeneous mixture. Hexamethyldisilazane was used to impart hydrophobic properties to fumed silica. While the vulcanizable compositions disclosed were cured by exposure to hot moist atmospheres such as steam, the hydrophobic fumed silica filler had to be free of moisture. A process which uniformly distributes moisture throughout hydrophilic fumed silica would be a desirable advancement in the art.